1. Field of the Invention
The present invention relates to a magnetic thin film that is used for a CPP-GMR (Current Perpendicular to the Plane Giant Magneto Resistance) element (hereinafter called a “CPP element”) which constitutes a thin-film magnetic head, and more particularly relates to the structure of a spacer layer.
2. Description of the Related Art
A CPP element is known as one of the magnetic field detecting elements which are used in a thin-film magnetic head. A CPP element has a pinned layer whose magnetization direction is fixed with respect to an external magnetic field, a free layer whose magnetization direction is changed in accordance with the external magnetic field, and a non-magnetic spacer layer that is sandwiched between the pinned layer and free layer (also called a “spacer layer”). In this specification, a stacked structure of layers that are comprised of a pinned layer, a non-magnetic spacer layer, and a free layer is called a “magnetic thin film.” A magnetic thin film is a central part of a CPP element for generating a change in magneto resistance by the GMR effect. The magnetic thin film forms a stacked structure of layers, which is referred to as a spin valve (hereinafter, referred to as a SV), together with other metallic layers. Sense current is configured to flow in a direction that is perpendicular to the film surfaces of the SV, i.e., the pinned layer, the spacer layer, and the free layer. A SV is sandwiched between a pair of shield layers. The shield layers functions as electrode layers for supplying sense current, as well as functions as a magnetic shield for the SV. In a CPP element, since the SV is physically connected with the shield layers, the CPP element has a high efficiency for heat radiation, and a large capacity for sense current. Further, the CPP element exhibits a larger electric resistance and a larger change in resistance in accordance with a decrease in cross section. Accordingly, the CPP element is more suitable for a narrow track width.
The SV in a CPP element is usually formed of metallic material because sense current flows in a direction that is perpendicular to the SV. Accordingly, electric resistance is significantly small. If electric resistance is small, then a change in electric resistance becomes small, and it is impossible to achieve a large MR ratio. For this reason, various techniques for increasing electric resistance have been disclosed. The specification of Japanese Patent Laid-Open Publication No. 2006-135253 discloses a technique to provide a region having large electric resistance and a region having small electric resistance in a spacer layer. Since most sense current flows through the region of smaller electric resistance, an effect can be achieved that is similar to the effect that would be obtained when the cross-section of the element is actually reduced, and increased electric resistance can be obtained. The region of small resistance is formed of Cu, Ag, Au, Pt, or the like. The region of large resistance is formed of oxide of Al, Mg, or the like. The specification of Japanese Patent Laid-Open Publication No. 2004-153248 discloses a technique to form a similar large resistance region and a small resistance region by using phase separation of metal alloy.
However, even a CPP element having a spacer layer that is provided with a region of large resistance and a region of small resistance, as mentioned above, does not exhibit a sufficiently large MR ratio in a low RA region. The RA value means a product of electrical resistance R of a SV to sense current and the minimum cross sectional area A that is taken in the plane of a film surface of the SV. When the RA value is large, S/N ratio is significantly reduced due to shot noises. Therefore, the RA value is preferably 0.35(Ω μm2) or less when the CPP element is applied to a magnetic head. However, when the RA value is 0.35(Ω μm2) or less, the MR ratio is in a range of as low as 4 to 5%. There is a need to further improve the MR ratio in order to put a head having 400 Gpsi or more to practical use.